Bottom antireflection coating color filter process for fabricating solid state image sensors

ABSTRACT

An image sensor system and methods of making such a system are described. The image sensor system includes a color filter array that is formed by a color filter process that incorporates a bottom antireflection coating. The bottom antireflection coating forms a protective layer that protects exposed areas of the active image sensing device structure during formation of the color filter array and, thereby, preserves the intrinsic transmission characteristics of the active image sensing device structure. The bottom antireflection coating also reduces degradation of metal structures (e.g., bonding pads) and pixel edges at the exposed surface of the active image sensing device structure. In addition, the bottom antireflection coating provides a reliable adhesive surface for the color filter array, substantially eliminating lifting of the color filter array resist structures. In some embodiments, the bottom antireflection coating also improves the optical transmission characteristics of one or more colors of the colors filter array.

This is a Divisional of application Ser. No. 09/938,394, filed on Aug.23, 2001 now U.S. Pat. No. 6,765,276, the entire disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to solid state image sensors that include colorfilter arrays that are formed by a bottom antireflection coating (BARC)color filter process, and methods of making the same.

BACKGROUND

In general, digital imaging systems, such as digital cameras, includeimage sensors (or simply imagers) for capturing images. Various types ofimage sensors have been developed, including charge-coupled device (CCD)image sensors and complementary metal-oxide semiconductor (CMOS) imagesensors. These devices typically include an array of pixels, each ofwhich contains a light-sensing element, such as an n+ to p-substratephotodiode, a virtual gate buried n-channel photodetector, or aphoto-gate detector, which defines a light-sensing region of an imagesensor. Image sensors also include circuitry for driving light signalsfrom the light-sensing elements to other process circuitry. CCD imagesensors typically include a photoelectric converter and chargeaccumulator for absorbing light from an object and collectingphoto-generated charges into signal charge packets. In addition, CCDimage sensors may include a charge transfer region for conveying chargepackets from the photoelectric converter and charge accumulator, and acharge-to-voltage signal converter for generating a voltage outputcorresponding to the signal charge packets that are transferred throughthe charge transfer region. CMOS image sensors typically include anarray of active pixel sensors and a row (register) of correlateddouble-sampling (CDS) amplifiers that sample and hold the output of agiven row of pixel sensors. In both CMOS and CCD image sensor systems,each pixel sensor accumulates charge during an optical integrationperiod in accordance with the light intensity reaching the relevantsensing area of the pixel sensor.

In color applications, each pixel sensor element typically receiveslight through a color filter that allows only a relatively narrowradiation wavelength range (e.g., the visible spectrum) to reach thepixel sensors of the image sensor. Multiple sets of color filterstypically are arranged in a pattern of pixel size mosaics or pixel widestripes. The color filters may be applied directly to the surface of animage sensor. Alternatively, the color filters may be formed on apassivation layer (see, e.g., U.S. Pat. No. 5,654,202), in which case aseparate masking step is required to expose the bonding pads of theimage sensor. The color filters typically are formed from a photoresiststructure that includes a layer for each filter color. A common colorfilter material is spin coated-, dyed-, or pigmented-photoresist. Thefilter colors for a given color filter set may be additive (e.g., red,green, blue) or subtractive (e.g., cyan, magenta, yellow), or acombination of both additive and subtractive.

The light collecting efficiency of an image sensor may be improved bydepositing a micro lens array over the CFA material of each pixelregion. A planarization layer that is highly transmissive in the imagingwavelength range also may be deposited between the color filter arrayand the micro lens material.

SUMMARY

The invention features a novel image sensor system and methods of makingsuch a system. In particular, the novel image sensor system includes acolor filter array that is formed by an inventive color filter processthat incorporates a bottom antireflection coating. The bottomantireflection coating forms a protective layer that protects exposedareas of the active image sensing device structure during formation ofthe color filter array and, thereby, preserves the intrinsictransmission characteristics of the active image sensing devicestructure. For example, the bottom antireflection coating protectssensitive areas of the active image sensing device structure againstdegradation that otherwise might be caused by exposure to developingsolutions that are used to pattern the color filter array. The bottomantireflection coating also reduces degradation of metal structures(e.g., bonding pads) and pixel edges at the exposed surface of theactive image sensing device structure. For example, the bottomantireflection coating reduces scumming and chemical reactions thatotherwise might occur at such metal structures and pixel edges as aresult of repeated exposure to color filter resist material and colorfilter developing solutions. In addition, the bottom antireflectioncoating provides a uniform adhesive surface for the color filter array,substantially eliminating lifting of the color filter array resiststructures. In some embodiments, the bottom antireflection coating alsoimproves the optical transmission characteristics of one or more colorsof the colors filter array.

In one aspect of the invention, a bottom antireflection coating isformed over an exposed surface of an active image sensing devicestructure, a color filter array is formed on the bottom antireflectioncoating, and exposed portions of the bottom antireflection coating aresubstantially removed.

Embodiments in accordance with this aspect of the invention may includeone or more of the following features.

The bottom antireflection coating may comprise a dyed organicfilm-forming material or a light-absorbing polymeric film-formingmaterial.

The bottom antireflection coating preferably has a thickness that isselected to improve optical transmission characteristics of one or morecolors of the color filter array. In addition, the bottom antireflectioncoating preferably is substantially transmissive to radiation in awavelength range of about 400 nm to about 700 nm.

In some embodiments, the color filter array comprises a plurality ofcolored photoresist structures.

Exposed portions of the bottom antireflection coating preferably areremoved substantially by a plasma etch process (e.g., a low-powerbuffered oxygen ash process). The plasma etch process preferably removesthe bottom antireflection coating at a substantially higher etch ratethan the color filter array.

In some embodiments, the bottom antireflection coating forms asubstantially continuous layer over the exposed surface of the activeimage sensing device structure before exposed portions of the bottomantireflection coating are substantially removed. The bottomantireflection coating may form a protective barrier over metalstructures at the exposed surface of the active image sensing devicestructure during formation of the color filter array.

The active image sensor device structure may be a complementarymetal-oxide-semiconductor (CMOS) image sensor or a charged-coupleddevice (CCD) image sensor.

In another aspect, the invention features an image sensor system thatincludes an active image sensing device structure, a color filter array,and a bottom antireflection coating that is disposed between the colorfilter array and a surface of the active image sensing device structure.

Other features and advantages of the invention will become apparent fromthe following description, including the drawings and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a portion of an image sensor system thatincludes a bottom antireflection coating disposed between a color filterarray and a top surface of an active image sensing device structure.

FIG. 2 is a flow diagram of a process of fabricating the image sensorsystem of FIG. 1.

FIG. 3 is a diagrammatic cross-sectional side view of an image sensorsystem being formed in accordance with the process of FIG. 2 after anactive image sensing device structure has been formed.

FIG. 4 is a diagrammatic cross-sectional side view of an image sensorsystem being formed in accordance with the process of FIG. 2 after abottom antireflection coating has been formed over an exposed surface ofthe active image sensing device structure of FIG. 3.

FIG. 5 is a diagrammatic cross-sectional side view of an image sensorsystem being formed in accordance with the process of FIG. 2 after acolor filter array has been formed on the bottom antireflection coatingof FIG. 4 and exposed portions of the bottom antireflection coating havebeen removed.

DETAILED DESCRIPTION

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

Referring to FIG. 1, in one embodiment, an image sensor system 10includes an active image sensing device structure 12, a color filterarray 14, and a bottom antireflection coating (BARC) layer 16 disposedbetween a top surface of active image sensing device structure 12 andcolor filter array 14. Image sensor system 10 also includes apassivation layer 18 and a conventional micro lens array 20 that isformed over passivation layer 18.

Referring to FIGS. 2, 3, 4 and 5, and initially to FIGS. 2 and 3, imagesensor system 10 may be formed as follows. An active image sensingdevice structure 12 initially is formed (step 32). Active image sensingdevice structure 12 may be a conventional CCD imaging device structureor a conventional CMOS imaging device structure. In general, activeimage sensing device structure 12 may include an array of pixels, eachof which contains a light-sensing element (e.g., a p-i-n photodiode, ann+ to p-substrate photodiode, a virtual gate buried n-channelphotodetector, or a photo-gate detector), and circuitry for drivinglight signals from the light-sensing elements to other processcircuitry.

As shown in FIG. 3, in one embodiment, active image sensing devicestructure 12 includes a plurality of pixel transistors 34 that areformed on a substrate 36, a number of metallization interconnect levels38 (not shown individually), and a final metallization level 40. Finalmetallization level 40 includes a bonding pad 42, a ground contact 44,and a pair of metallization contacts 46, 48. Final metallization level40 also includes a polished oxide (e.g., silicon oxide) layer 50, asilicon nitride layer 52, and a number of tungsten-filled contact vias54, 56, 58 extending through oxide and silicon nitride layers 50, 52down to a respective contact 44–48. A Ti/TiN barrier (or liner) layermay be deposited on the surfaces of contact vias 54–58 before thetungsten plugs are formed. A via 59 is formed through oxide and siliconnitride layers 50, 52 to expose bonding pad 42. A pixel metallization60, 62 may be formed over each of the tungsten plugs 56, 58 that areconnected to metallization contacts 46, 48. Patterned n-type amorphoussilicon layers 64, 66 are formed over the pixel metallizations 60, 62.An intrinsic amorphous silicon layer 68 is formed over n-type amorphoussilicon layers 64, 66, and a p-type amorphous silicon layer 70 is formedover the intrinsic amorphous silicon layer 68. A transparent conductivelayer 72 extends over p-type amorphous silicon layer 70 and contacts thetungsten plug connected to ground contact 44. An opaque metal layer 74extends over a portion of transparent conductive layer 72 to bridgetransparent conductive layer 72 at the ground contact perimeter, blockslight at pixel borders, and surrounds the pixel array.

As shown in FIG. 4, after active image sensing device structure 12 isformed (step 32; FIG. 2), a BARC layer 16 is deposited over an exposedsurface of active image sensing device structure 12 (step 76; FIG. 2).In general, BARC layer 16 may be formed from any conventional BARCmaterial, including a dyed organic film-forming BARC material or alight-absorbing polymeric film-forming BARC material. In someembodiments, BARC layer 16 preferably is substantially absorptive ofradiation in the wavelength range used to pattern color filter array 14and is substantially transmissive to radiation in the wavelength rangeto be imaged by image sensor system 10 (e.g., the visible radiationspectrum). BARC layer 16 may be formed from an organic film-formingmaterial or a polymeric film-forming material. In one embodiment, BARClayer 16 is a photoresist-based antireflective coating that issubstantially transmissive to radiation in a wavelength range of about400 nm to about 700 nm (e.g., a Shipley AR2-600 antireflection coating,which is available from Shipley Company, L.L.C. of Marlborough, Mass.,U.S.A.). In this embodiment, BARC layer 16 may be applied by aconventional spin-coater operating at 2000 rpm during deposition and at4790 rpm during spreading; the resulting BARC layer 16 has a thicknessof about 60 nm. After deposition, BARC layer 16 is exposed to a 60second proximity bake at 205° C. on a DNS track.

In general, BARC layer 16 may have a thickness that is selected toimprove the optical transmission characteristics of one or more colorsof color filter array 14. In particular, the BARC layer thickness may beselected so that the peak transmission at one or more target radiationwavelengths is increased relative to device structures that do notinclude BARC layer 16. The target radiation wavelengths may correspondto the wavelengths of peak pixel sensitivity for each color of the colorfilter set of the completed image sensor system 10. The opticaltransmission characteristics for each color may be modeled based uponthe refraction indices of BARC layer 16 and the other layers of imagesensing device structure 12. The thicknesses of BARC layer 16 and thelayers of color filter array 14 may be varied within specified thicknessranges until the peak transmissions for one or more of the targetradiation wavelengths of the color filter set are optimized. In theillustrated embodiment, it has been discovered that a BARC layerthickness of approximately 60 nm improves the optical transmissioncharacteristics for each of the colors of an RGB color filter array atthe target radiation wavelengths of 620 nm for red, 540 nm for green,and 460 nm for blue. In general, the BARC layer thickness should berelatively thin (e.g., less than approximately 200 nm) so that portionsof BARC layer 16 may be removed relatively easily to expose bonding pad42 and other device structures during the BARC layer removal processstep (described below in connection with process step 90 of FIG. 2).

Referring to FIG. 5, a patterned color filter array 14 is formed on BARClayer 16 (step 82; FIG. 2). BARC layer 16 provides a reliable adhesivesurface for the color filter array 14 and, thereby, substantiallyeliminates lifting of the color filter array resist structures. Colorfilter array 14 may be a conventional color filter array that is formedfrom a plurality of colored photoresist structures that are arranged ina pattern of pixel size mosaics or pixel wide stripes. For example,color filter array 14 may be polymer color filter array that is formedby successive deposition of the colored photoresist layers of aparticular color set (e.g., red/green/blue or cyan/magenta/yellow). Amasking or etching process may be used to form a respective color filterat a selected pixel location. In general, a pixel color filter may beformed from a single polymer layer containing one or several dyes, or byseveral polymer layers, each of which contains one or more dyes. Asshown in FIG. 1, the resulting color filter array 14 includes adistinct, spatially separated filter region 84, 86, 88 for each filtercolor (e.g., red, green and blue in the illustrated embodiment), eachregion 84, 86, 88 corresponding to a different color pixel of imagesensor system 10. By this arrangement, light at different wavelengthsmay be sampled separately so that color-separated images may be formed.

As explained above, BARC layer 16 protects exposed areas of the activeimage sensing device structure 12 during formation of the color filterarray 14 and, thereby, preserves the intrinsic transmissioncharacteristics of the active image sensing device structure 12. Forexample, BARC layer 16 protects sensitive areas of active image sensingdevice structure 12 against degradation that otherwise might be causedby exposure to the developing solutions that are used to pattern thecolor filter array 14. The bottom antireflection coating also reducesdegradation of metal structures (e.g., bonding pad 42) and pixel edgesat the exposed surface of the active image sensing device structure 12.For example, BARC layer 16 reduces scumming and chemical reactions thatotherwise might occur at such metal structures and pixel edges as aresult of repeated exposure to color filter resist material and colorfilter developing solutions.

Next, exposed portions of BARC layer 16 are removed (step 90; FIG. 2).BARC layer 16 may be removed in a conventional plasma etch system (e.g.,a LAM590 plasma etch system available from LAM Research Corporation ofFremont, Calif.). In one embodiment, a low-power buffered ash (e.g., aHe/O₂ ash) may be used to remove BARC layer 16. The plasma etch processpreferably removes BARC layer 16 at a substantially higher etch ratethan the color filter array 14. It has been discovered that the processof removing portions of BARC layer 16 also improves the opticaltransmission characteristics of color filter array 14 by removing (orcleaning) portions of color filter array 14 that are stained duringformation of color filter array 14. For example, during formation of anRGB color filter array, the array of filters for the first color (e.g.,red) that is formed may be stained by the subsequent color resistprocessing steps that are used to form the filter arrays for theremaining colors (e.g., blue and green). In addition, the array offilters for the second color (e.g., blue) that is formed may be stainedby the subsequent color resist processing steps that are used to formthe filter array for the remaining color (e.g., green). Such stainingreduces the optical transmission through the stained color filterarrays. The process of removing portions of BARC layer 16, however,cleans the surfaces of the stained color filter arrays and, thereby,improves their optical transmission characteristics.

Additional structures, including passivation layer 18 and micro lensarray 20, may be formed after the exposed portions of BARC layer 16 havebeen removed (step 92; FIG. 2). These additional structures may beformed in accordance with conventional device fabrication processes.

Other embodiments are within the scope of the claims.

1. A method of fabricating an image sensor, comprising: forming a bottomantireflection coating over an exposed surface of an active imagesensing device structure; forming a color filter array on the bottomantireflection coating; and substantially removing exposed portions ofthe bottom antireflection coating; wherein the active image sensingdevice structure comprises an array of light sensing elements, formingthe color filter array comprises forming an array of color filters eachdisposed over a respective light sensing element such that light travelsfrom each color filter to a respective light sensing element through arespective light transmission path substantially transmissive toradiation in a visible wavelength range, forming the bottomantireflection coating comprises forming the bottom antireflectioncoating with a thickness less than approximately 200 nm, and after theremoving, remaining portions of the antireflection coating are disposedin each light transmission path between the color filter array and theactive image sensing device structure.
 2. The method of claim 1, whereinthe bottom antireflection coating comprises a dyed organic film-formingmaterial.
 3. The method of claim 1, wherein the bottom antireflectioncoating comprises a light-absorbing polymeric film-forming material. 4.The method of claim 1, wherein the bottom antireflection coating issubstantially transmissive to radiation in a wavelength range of about400 nm to about 700 nm.
 5. The method of claim 1, wherein the colorfilter array comprises a plurality of colored photoresist structures. 6.The method of claim 1, wherein exposed portions of the bottomantireflection coating are removed substantially by a plasma etchprocess.
 7. The method of claim 6, wherein the plasma etch process is alow-power buffered oxygen ash process.
 8. The method of claim 1, whereinthe bottom antireflection coating forms a substantially continuous layerover the exposed surface of the active image sensing device structurebefore exposed portions of the bottom antireflection coating aresubstantially removed.
 9. The method of claim 1, wherein the bottomantireflection coating forms a protective barrier over metal structuresat the exposed surface of the active image sensing device structureduring formation of the color filter array.
 10. The method of claim 1,wherein the active image sensor device structure comprises acomplementary metal-oxide-semiconductor (CMOS) image sensor.
 11. Amethod of fabricating an image sensor, comprising: forming a bottomantireflection coating over an exposed surface of an active imagesensing device structure: forming a color filter array on the bottomantireflection coating; and substantially removing exposed portions ofthe bottom antireflection coating; wherein the bottom antireflectioncoating has a thickness selected to improve an optical transmissioncharacteristic of one or more colors of the color filter array.
 12. Amethod of fabricating an image sensor, comprising: forming a bottomantireflection coating over an exposed surface of an active imagesensing device structure; forming a color filter array on the bottomantireflection coating; and substantially removing exposed portions ofthe bottom antireflection coating, wherein exposed portions of thebottom antireflection coating are removed substantially by a plasma etchprocess that removes the bottom antireflection coating at asubstantially higher etch rate than the color filter array.
 13. A methodof fabricating an image sensor, comprising: forming a bottomantireflection coating over an exposed surface of an active imagesensing device structure; forming a color filter array on the bottomantireflection coating; and substantially removing exposed portions ofthe bottom antireflection coating, wherein, after the removing, thebottom antireflection coating is present only in regions directly undercolor filter array material.
 14. A method of fabricating an imagesensor, comprising: forming a bottom antireflection coating over anexposed surface of an active image sensing device structure, wherein thebottom antireflection coating has a thickness of about 60 nm; forming acolor filter array on the bottom antireflection coating; andsubstantially removing exposed portions of the bottom antireflectioncoating.